A traditional way of constructing a turbine diaphragm is to mount an annulus of aerofoil blades between an inner ring and an outer ring. Each blade is formed as part of a blade unit in which the blade extends between an inner platform and an outer platform, the blade unit being machined as a single component. Each platform is in the form of a segment of a cylinder so that when the annulus of blade units is assembled the inner platforms combine to create an inner cylinder and the outer platforms combine to create an outer cylinder. The outer platforms are welded to an outer ring that provides support and rigidity to the diaphragm.
The inner platforms are welded to an inner ring that prevents axial deflection of the turbine blades. In some known variants, the inner and outer rings are each divided into two semicircular halves along a plane that contains the axis of the diaphragm and passes between blade units so that the entire diaphragm can be separated into two parts for assembly around the rotor of the turbo-machine. The two halves of the outer ring can be bolted together when the diaphragm is assembled. The two halves of the inner ring are typically held in place by being welded to the blade units, which in turn are welded to the outer ring.
U.S. Patent Application Publication No. 2008/0170939 and published International Patent Application WO 2011/018413 disclose a compact turbine diaphragm that does away with the inner ring, thereby saving the cost of manufacturing that component and the cost of welding it to the blade units. The inner platforms are made to interlock in such a manner that the inner cylinder created by them serves the purpose of the inner ring. During assembly, the blade units become subject to a torque that pre-stresses them and helps to increase the rigidity of the diaphragm.
Though the diaphragms assembled, according to the '939 publication, require no welding operation at the inner ring, the outer ring is still welded against the outer diaphragm ring. The welding of a diaphragm is a complex and expensive process, which additionally requires a post-welding heat-treatment and final machining process to correct distortions. In addition only a small number of factories are qualified to manufacture these diaphragms. A mechanical assembly would reduce cost by dispensing with the weld process and in addition would permit sourcing of the assembly from a wider range of suppliers.
Such problems are for example partly addressed in U.S. Patent Application Publication No. 2007/0292266. In the method disclosed, the high heat input welding is replaced by a low heat input type or shallow weld. A proposal for assembling a diaphragm without welding is also described for example in U.S. Pat. No. 7,179,052.
Given the state of the art, there can be seen a constant demand for the industry to facilitate the assembly of turbine diaphragms, avoiding welding as far as possible while maintaining the required mechanical stability of the assembled turbine and its parts.